2,171 research outputs found
Periodic-orbit approach to the nuclear shell structures with power-law potential models: Bridge orbits and prolate-oblate asymmetry
Deformed shell structures in nuclear mean-field potentials are systematically
investigated as functions of deformation and surface diffuseness. As the
mean-field model to investigate nuclear shell structures in a wide range of
mass numbers, we propose the radial power-law potential model, V \propto
r^\alpha, which enables a simple semiclassical analysis by the use of its
scaling property. We find that remarkable shell structures emerge at certain
combinations of deformation and diffuseness parameters, and they are closely
related to the periodic-orbit bifurcations. In particular, significant roles of
the "bridge orbit bifurcations" for normal and superdeformed shell structures
are pointed out. It is shown that the prolate-oblate asymmetry in deformed
shell structures is clearly understood from the contribution of the bridge
orbit to the semiclassical level density. The roles of bridge orbit
bifurcations in the emergence of superdeformed shell structures are also
discussed.Comment: 20 pages, 23 figures, revtex4-1, to appear in Phys. Rev.
Competition between singlet and triplet pairings in Na_xCoO_2 yH_2O
We discuss the pairing symmetry of a cobaltate superconductor
NaCoO HO by adopting an effective single band model that
takes into account the hole pockets, as discussed in our previous paper
[to appear in Phys. Rev. Lett.] Here we consider the off-site repulsions in
addition to the on-site repulsion considered in our previous study. We show
that the spin-triplet f-wave pairing proposed in our previous study is robust
to some extent even in the presence of off-site repulsions. However, f-wave
pairing gives way to singlet pairings for sufficiently large values of off-site
repulsions. Among the singlet pairings, i-wave and extended s-wave pairings are
good candidates which do not break time reversal symmetry below in
agreement with the experiments.Comment: 12 page
Three-orbital study on the orbital distillation effect in the high Tc cuprates
Our recent study has revealed that the mixture of the dz2 orbital component
into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have
also shown that applying hydrostatic pressure enhances Tc due to smaller mixing
of the Cu4s component. We call these the "orbital distillation" effect. In our
previous study, the 4s orbital was taken into account through the hoppings in
the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and
4s orbitals are all considered explicitly. The present study reinforces our
conclusion that smaller 4s hybridization further enhances Tc.Comment: 4 pages, 2 figures, submitted as a proceeding of ISS2012(Tokyo
Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation
By coupling a first-principles, spin-density functional calculation with an
exact diagonalization study of the Hubbard model, we have searched over various
functional groups for the best case for the flat-band ferromagnetism proposed
by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic
polymers of five-membered rings. The original proposal (poly-aminotriazole) has
turned out to be the best case among the materials examined, where the reason
why this is so is identified here. We have also found that the ferromagnetism,
originally proposed for the half-filled flat band, is stable even when the band
filling is varied away from the half-filling. All these make the ferromagnetism
proposed here more experimentally inviting.Comment: 11 pages, 13figure
Remarks on the multi-species exclusion process with reflective boundaries
We investigate one of the simplest multi-species generalizations of the one
dimensional exclusion process with reflective boundaries. The Markov matrix
governing the dynamics of the system splits into blocks (sectors) specified by
the number of particles of each kind. We find matrices connecting the blocks in
a matrix product form. The procedure (generalized matrix ansatz) to verify that
a matrix intertwines blocks of the Markov matrix was introduced in the periodic
boundary condition, which starts with a local relation [Arita et al, J. Phys. A
44, 335004 (2011)]. The solution to this relation for the reflective boundary
condition is much simpler than that for the periodic boundary condition
Spin-triplet superconductivity in repulsive Hubbard models with disconnected Fermi surfaces: a case study on triangular and honeycomb lattices
We propose that spin-fluctuation-mediated spin-triplet superconductivity may
be realized in repulsive Hubbard models with disconnected Fermi surfaces. The
idea is confirmed for Hubbard models on triangular (dilute band filling) and
honeycomb (near half-filling) lattices using fluctuation exchange
approximation, where triplet pairing order parameter with f-wave symmetry is
obtained. Possible relevance to real superconductors is suggested.Comment: 5 pages, 6 figures, RevTeX, uses epsf.sty and multicol.st
Possible high superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets
We propose that if there are two small pocket-like Fermi surfaces, and the
spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the
two pockets, the spin-singlet superconductivity mediated by spin fluctuations
may have a high transition temperature. Using the fluctuation exchange
approximation, this idea is confirmed for the Hubbard on a lattice with
alternating hopping integrals, for which is estimated to be almost an
order of magnitude larger than those for systems with a large connected Fermi
surface.Comment: 5 pages, uses RevTe
Stability of Ferromagnetism in Hubbard models with degenerate single-particle ground states
A Hubbard model with a N_d-fold degenerate single-particle ground state has
ferromagnetic ground states if the number of electrons is less or equal to N_d.
It is shown rigorously that the local stability of ferromagnetism in such a
model implies global stability: The model has only ferromagnetic ground states,
if there are no single spin-flip ground states. If the number of electrons is
equal to N_d, it is well known that the ferromagnetic ground state is unique if
and only if the single-particle density matrix is irreducible. We present a
simplified proof for this result.Comment: accepted for publication in J. Phys.
Multipole expansion for magnetic structures: A generation scheme for symmetry-adapted orthonormal basis set in crystallographic point group
We propose a systematic method to generate a complete orthonormal basis set
of multipole expansion for magnetic structures in arbitrary crystal structure.
The key idea is the introduction of a virtual atomic cluster of a target
crystal, on which we can clearly define the magnetic configurations
corresponding to symmetry-adapted multipole moments. The magnetic
configurations are then mapped onto the crystal so as to preserve the magnetic
point group of the multipole moments, leading to the magnetic structures
classified according to the irreducible representations of crystallographic
point group. We apply the present scheme to pyrhochlore and hexagonal ABO3
crystal structures, and demonstrate that the multipole expansion is useful to
investigate the macroscopic responses of antiferromagnets
Breakdown of a Mott insulator -- non-adiabatic tunneling mechanism
Time-dependent nonequilibrium properties of a strongly correlated electron
system driven by large electric fields is obtained by means of solving the
time-dependent Schr\"odinger equation for the many-body wave function
numerically in one dimension. While the insulator-to-metal transition depends
on the electric field and the interaction, the metallization is found to be
described in terms of a universal Landau-Zener quantum tunneling among the
many-body levels. These processes induces current oscillation for small
systems, while give rise to finite resistivity through dissipation for larger
systems/on longer time scales.Comment: 5 pages, 5 figures, version to appear in Phys.Rev.Let
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